1. Field of the Invention
This invention generally relates to RF circuits and more specifically to an RF signal divider that is operable over a broad bandwidth.
2. Description of Related Art
Wireless RF applications in the 200 to 3000 MHz range, particularly in the 800 to 1000 MHz and 1900 to 2400 MHz bands, have become wide spread in recent years. These are frequencies of choice for wireless telephones and similar communication applications. Particular effort has been directed to the development of the RF transmitting and receiving facilities for such applications including wireless telephone repeaters or cells.
In many of these applications it is necessary to split or divide a low-level received RF input signal into a high-power RF output signal is transmitted to other cells or telephones. In a repeater or cell application, for example, a received signal from an antenna may be amplified in a preamplifier stage and then split and fed through multiple parallel power amplifiers to be recombined as a RF output signal.
A number of RF signal dividers and combiners have been proposed for such applications. For example, the well known Wilkinson circuit uses transmission lines at a characteristic impedance to convey signals to different ports. The ports are tied through transmission lines to a common node. The transmission lines may be a quarter wavelength (xcex/4) in length.
Variations on the Wilkinson circuit have been proposed. For example, U.S. Pat. No. 4,463,326 (1984) to Hom discloses a microwave combiner and divider circuit that uses microstrips, or strip line, media. A common signal port feeds a division point through a capacitive stub and two-stage ring-type impedance matching circuit. Multiple circuit traces emanate from the division point to N individual branch ports. Strip line circuitry between the division point and branch ports provides compensation for phase reversal. Resistors are provided for branch port isolation.
U.S. Pat. No. 4,968,958 (1990) to Hoare discloses an RF signal divider that also uses strip line components to produce various transmission lines. Specifically, a metallic layer in a divider tapers laterally outwardly from an input port and then splits into at least two tapering conductors for the output ports. A resistive network provides isolation.
U.S. Pat. No. 4,893,093 (1990) to Cronauer et al. discloses a splitter in which a high frequency input signal is applied to a plurality of amplifiers. First transmission lines connect the input and each of the amplifiers. Each transmission line can be switched between high and low impedance levels. A balanced resistor network preferably interconnects the first transmission lines. Second transmission lines shunt the first transmission lines. The impedance of each second transmission line can be altered to a predetermined percentage of the circuit input impedance. A control circuit switches the various transmission lines so that the impedance of the antenna remains balanced no matter how many of the first transmission lines are in the high impedance state.
Another approach to dividing RF signals involves the utilization of transformers and hybrid couplers. U.S. Pat. No. 5,264,810 (1993) to Sager et al. discloses one such circuit in the form of a three-way divider in which signals from an amplifier energize three transformer primary windings in series. First secondary windings couple RF signals to three output amplifiers. Second secondary transformer windings and resistors maintain signal balance.
U.S. Pat. No. 5,313,174 (1994) to Edwards discloses a power splitter that maintains quadrature phase over the entire bandwidth of an input signal. This structure uses a combination of strip line technology and hybrid couplers to provide the appropriate output signals in phase quadrature.
Other patents utilize a combination of transmission lines and resistors. For example, U.S. Pat. No. 5,021,755 (1991) to Gustafson discloses an N-way signal splitter with a printed circuit board geometry and orthogonal resistive components. This circuit includes an impedance matching input circuit and a splitter circuit using both transmission lines and resistors.
U.S. Pat. No. 5,872,491 (1999) to Kim et al. disclose a Wilkinson-type power divider/combiner that has a selective switching capability. The switchable power divider/combiner includes N first switches connecting N input/output transmission lines to a common junction and N second switches connecting N isolation resistors coupled to the N input/output transmission lines to a common node. The activation of each pair of the first and second switches to a closed or opened switch position controls the operating mode.
Generally speaking prior art RF dividers are designed for operation at a fixed frequency or, at most, over a limited design frequency range or band. Physical size limits in such applications constrain the range or band. Within the design frequency range such RF dividers operate with good insertion loss and isolation characteristics. If such an RF divider operates at a frequency outside the design frequency range, however, these characteristics deteriorate. This deterioration is due in large part to the frequency dependency of power division and isolation mechanisms that the prior art RF signal dividers use.
For example, the common node impedance of each branch of a Wilkinson-type RF signal divider is greater than the characteristic impedance of the circuit in which the RF signal divider is used. This produces an impedance mismatch at the entrance to the branch that is frequency dependent. While a specific design can maintain this impedance mismatch within acceptable limits over the design frequency range, at some operating frequency outside the design frequency range the impedance mismatch will become unacceptable. More specifically, the impedance mismatch determines a proportional fraction of the power at a common node can transfer into the branch. This is an inverse relationship. Consequently, as the number of branches from the common node increase, the higher the impedance mismatch, the lower the proportional fraction and the less power transferred.
In Wilkinson-type RF signal dividers resistive paths provide isolation among the signals that transfer over the various divider branches. These resistive paths are typically selected to introduce appropriate phase differences in signals that provide signal cancellation. However, the effect of these paths is also frequency dependent. Such paths provide good isolation over a limited design frequency range. However, as the operating frequency for such an RF signal divider moves outside the design frequency range, the signal isolation reduces.
However, there are situations in which it is desirable to use a single compact RF divider in variable frequency applications where the frequency range is quite large. There are other situations in which is desirable to use multiple RF signal dividers of one type in different fixed frequency applications with multiple fixed operating frequencies. These prior art RF signal dividers do not operate with uniformly good insertion loss and isolation characteristics in either of the foregoing situations. That is, in use such prior art RF signal dividers, that are subject to a size constraint, exhibit marked decreases in insertion loss and isolation characteristics in situations requiring either a single RF divider to operate over a wide frequency range or a single RF divider to operate at different fixed frequencies spread across a wide frequency range.
Therefore, it is an object of this invention to provide a compact RF signal divider that can be used in systems operating over a wide frequency range.
Another object of this invention is to provide an RF signal divider that can operate an any frequency in a wide range of frequencies with uniform insertion loss.
Still another object of this invention is to provide an RF signal divider that can operate at any frequency in a wide range of frequencies with uniformly high isolation.
In accordance with this invention an RF signal divider converts a received single RF input signal to a plurality of RF output signals of equal amplitude and phase. The RF divider comprises an input impedance transformer for receiving the RF input signal and a resistive path for each of the plurality of RF output signals. Each resistive path conveys its respective RF signal from the input impedance transformer as an RF output signal.
In accordance with another aspect of this invention, an RF divider comprises an RF input connection for receiving RF signals from an RF source and a plurality of RF output connections. A resistive path extends from each RF output connection to a common node. An RF impedance transformation path extends between the common node and the RF input connection. RF signals from the RF input connection pass through the RF transformation path and divide among the plurality of resistive paths to provide a plurality of RF signals of equal amplitude and phase at the RF output connections.
In accordance with yet another aspect of this invention, an RF signal divider comprises an RF input connection for receiving RF signals from an RF source and a plurality of RF output connections. A resistive path constitutes a sole RF path from each RF output connection to a common node. An RF impedance transformation path constitutes a sole RF path between the common node and the RF input connection. Signals at the RF input connection pass through the RF transformation path and divide among the plurality of resistive paths to provide a plurality of RF signals of equal amplitude and phase at the RF output connections.